Power switching transistors in power conversion circuits are often connected to an inductive load such as an inductor used in filter circuits or a transformer leakage inductance. At high frequency operation, the turn off time delay due to stored charges and the collector current fall time of a power switching transistor limit its maximum operating frequency. By minimizing the storage and fall times, the operating frequency as well as the circuit efficiency can be increased. A conventional turn off method employs a circuit to reverse bias the emitter junction to effect the turn off function. Once reverse voltage is applied to the base-emitter current flow, the collector to emitter current flow remains unchanged due to the stored charge in the base collector region. Due to the lateral resistance in the base region, the collector to emitter current continues to flow through a much smaller emitter area significantly increasing the current density per unit area. Hence there are localized larger internal temperature increases concurring with the localized increased current density. To make the operation more difficult, an inductive circuit forces the collector to emitter voltage to its clamped value (very high) at the end of the storage time since at that moment the collector current begins to fall. This adds to the stress of the power transistor and very frequently is a cause for its destruction. In order to improve the switching performance of a power transistor, it is desirable to provide circuitry to both minimize the turn off time and minimize the current crowding within the emitter base region.
Solutions to the aforementioned problems have included using a bigger power transistor having a larger base-emitter junction area thereby reducing internal current density due to crowding during turn off; and the use of specialized turn off loss reduction networks connected to the power transistor which are operative to limit the rise in voltage across the power transistor during turn off. All of these solutions add to the cost of the circuit by requiring added circuitry not essential to the basic power processing function of the power supply and which are only utilized for protection of the power switching transistor.